Jeremy Laurance: 'Even the doctor can't tell us just how long we've got left to live'

The diplomatic row over the release of Abdelbaset Ali al-Megrahi, the Lockerbie bomber, casts interesting light on how doctors judge the length of our lives. The Libyan national, aged 58, was released on compassionate grounds from a life sentence on 20 August last year after being diagnosed with terminal prostate cancer. It was said that three months was a "reasonable estimate" of how long he had left to live. Almost a year later, he is still alive.

Doctors have to be optimists. "Oh dear, it looks bad" is not a helpful response for any patient, however grave their condition. Keeping up the spirits of those they minister to is a fundamental part of care.

But they must also avoid unduly raising expectations, only to have them dashed. When asked how long a patient with a terminal illness has got, most doctors will play it cautiously. Better to say three months and have them live longer than to say six months and witness the shock of a family surprised by an unexpectedly sudden death.

In Megrahi's case, the situation was complicated by the public interest in having him die sooner rather than later. Normally, no one would be likely to protest if a patient lived longer than expected. Here, it mattered. Perhaps his doctors, rightly judging that their duty was to the patient in front of them, failed to recognise the public impact of their advice. But the most important lesson from the case is what was meant by that "three months to live" estimate. Did the doctors really put it like that? I wonder.

The best estimate of how long a patient has to live is based on the median survival of patients with their condition. But being a median it means half die sooner and half live longer – some much longer. The statistical distribution has a long tail.

The evolutionary biologist Stephen Jay Gould wrote a wonderful essay, "The Median isn't the Message", which is worth reading by anyone diagnosed with cancer. He was diagnosed with abdominal mesothelioma, a rare cancer caused by asbestos, at the age of 40 and told the median survival for his type of cancer was eight months. That was in 1982. He lived for another 20 years, dying in 2002 of an unrelated cancer. In his essay, Gould described how our emphasis on "clear distinctions and separated immutable entities" leads us to view statistical measures, wrongly, as "hard realities", when they are anything but. "All evolutionary biologists know that variation itself is nature's only irreducible essence."

He concludes with a message of defiance to all those who try to predict the end. "It has become, in my view, a bit too trendy to regard the acceptance of death as something tantamount to intrinsic dignity. Of course I agree with the preacher of Ecclesiastes that there is a time to love and a time to die... For most situations, however, I prefer the more martial view that death is the ultimate enemy – and I find nothing reproachable in those who rage mightily against the dying of the light."